Figure 4b. Petrogenetic grid based on the thermodynamic data set of Spear and Cheney (unpublished). The grid is an image map and images of calculated AFM diagrams can be viewed by clicking the mouse on P-T regions of the grid. Only one AFM diagram has been calculated for each P-T region, so changes in mineral composition across divariant regions cannot be examined. In the Gibbs program implementation, calculation is done for every P-T point specified (with a resolution of 0.2 degrees and 1 bar), so changes of mineral composition can be readily viewed across fields. See movie in Figure 5 to examine changes of Fe/Mg in a single divariant field. Note that in some P-T fields, the Al2SiO5 polymorph (Ky, And, Sil) shown on the AFM diagram will not be correct based on the P-T point clicked, but the magenta dot will show the correct P-T location for the specified polymorph.

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